The present invention relates to lasers, and, more particularly, to lasers which are tunable by means of filtered feedback.
Lasers emit electromagnetic radiation, "light" herein, over a characteristic bandwidth. A laser medium is often cable of lasing at a plurality of wavelengths. For example, the gain medium of an infrared semiconductor laser can support emissions over a range of roughly 500 Angstroms (.ANG.). In applications requiring laser emissions at a single predetermined wavelength, it is often necessary to tune a laser.
Tuning can be accomplished in principle by filtering light emitted by a laser source and feeding the filtered light back to the laser cavity. This feedback can have the effect of favoring resonance at the filtered bandwidth, which thus can dominate the laser output. Accordingly, wavelength selective feedback can cause the laser to oscillate at a single wavelength. Tuning of the laser is accomplished by controlling the wavelength of the filtered feedback signal.
Such feedback has been provided in the case of laser diodes by the use of feedback from an "external cavity". In a bulk optic external cavity laser, the output is collimated with a lens and sent to an angled diffraction grating. A narrow band of wavelengths is diffracted from the grating back along the path of the incident beam. The reflected beam is then coupled back into the laser cavity through the lens. The angle of the grating can be varied to control the wavelength of the light fed back to the laser. Accordingly, the laser can be tuned by rotating the diffraction grating.
The bulk optics approach has several disadvantages, especially in the context of diode-based lasers. Due to the small mode diameter of a laser diode, usually less than 4 microns (.mu.m), it is difficult to couple the diffracted signal stably into the laser cavity.
This stability problem is exacerbated when tuning the laser since the grating angle must be varied without inducing any other rotations or offsets to the grating. Decoupling tuning angle motion from other grating motions requires very stringent mechanical tolerances.
Due to the sensitive coupling, mechanical stability is also a problem. Shocks or bumps can disturb the optical coupling if great care is not taken in the mechanical design. Also, considerable space is required for the external grating and lens, making the assembly rather bulky.
The problems due to coupling are partially addressed by coupling the laser diode to an optic fiber which can serve as the external cavity. Where the filtering of light in the fiber involves removing the light and then reintroducing the filtered light into the fiber, the problems with coupling sensitivity are simply translated from one interface to another. However, the problems with coupling have been alleviated by the development of filters which operate on the light without removing it from the fiber.
The use of such an in-line filter for line-narrowing of a laser has been demonstrated by E. Brinkmeyer, W. Brennecke, M Zurn and R. Ulrich, as reported in "Fibre Bragg Reflector for Mode Selection and Line-Narrowing of Injection Lasers", Electronic Letters, Vol. 22, No. 3, Jan. 30, 1986, pp. 134-135. Brinkmeyer et al. disclose a laser diode coupled to an optic fiber with a reflective grating filter attached. The grating filter is disposed over a side-polished region of the fiber so that the evanescent field of light transmitted along the fiber core can interact with the periodic grating structure. The periodicity of the grating determines the wavelength of the reflected light, which in turn determines the narrowed resonance band of the laser diode.
The grating for such a filter can be fabricated using well-known holographic techniques. The interference front generated by a coupled pair of collimated laser beams can produce a series of generally parallel interference lines. These lines can be used to expose a photoresist coated substrate. The exposed substrate can be processed so that the interference pattern is represented as ridges on the finished grating.
The disclosed laser and fiber optic feedback system is limited, however, in that variable tuning is not provided. Thus, to select different output bands, the filter would have to be replaced by another with a different periodicity. To obtain a flexible tuning range and precision, a user would have to have a number of calibrated gratings available, along with a convenient means for exchanging them. Spectral sweeps, for example, are largely precluded.
A very limited form of tunability in a laser with a fiber grating external cavity is reported by C.A. Park et al., in "Single-Mode Behavior of a Multi-Mode 1.55.mu.m Laser with a Fibre Grating External Cavity", Electronics Letters, Vol. 22, No. 21, Oct. 9, 1986, pp. 1132-1133. Here, tuning was accomplished both by varying the refractive index of oil between a grating and a side-polished fiber and by thermally expanding the grating to increase its spatial periodicity. However, neither method provided a tuning range much greater than the bandwidth selected by the grating filter.
Furthermore, these approaches are disadvantageous because of the complexities and side effects involved. Changing the refractive index of the oil generally requires flushing out oil of one refractive index and replacing it with an oil of another refractive index. The hydraulics required for real-time tuning are impractical. Varying the temperature of the grating over the 40.degree. C. temperature range required to achieve even narrow band tuning is expensive to implement, especially given the need to control side effects of the temperature changes on other components of the laser system.
It is also possible to tune a fiber filter over a narrow band by rotating the filter relative to the fiber. However, the quality of the reflected signal diminishes with rotation angle, so this approach is self-limiting and self-compromising.
While narrow band tuning can preferentially select a given band while rejecting adjacent bands, the capability to select the given band is largely absent or severely limited. What is needed is a broadband tunable laser in which the given band can be selected over a range large compared to that defined by a single band and its immediate neighbors. Preferably, a laser should be tunable over the bandwidth of the laser medium, or at least over a bandwidth comparable to that of the laser medium.